1. Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus, and a shading correction method.
2. Description of the Related Art
A conventional image reading apparatus that optically reads a document so as to generate image data is known. In the image reading apparatus, due to various factors including variation in the sensitivity of a plurality of pixels of an image sensor, variation in the amount of light transmitted through a lens, variation in the illumination intensity of an illumination/optical system causing light intensity distribution in the main scanning direction or being caused by deterioration thereof, the output from a light receiving element to each corresponding pixel does not always have a constant or uniform value; therefore, shading correction is performed.
In the shading correction, before reading a document, a white reference member, being arranged near a document placement unit on which a document is placed, is irradiated with light from a light source, and the reflected light is read by a light receiving element so that white level reference data is obtained. During the shading correction, shading data is generated in reference to the obtained white level reference data and, by using the generated shading data, shading correction is performed on image data on the document that is read after the white reference member is read; thus, the image data is normalized.
In some image reading apparatuses, a document is conveyed by an automatic document feeder (ADF) and read by an optical reading system that is fixed at a predetermined position (hereafter, referred to as ADF reading).
With the image reading apparatus that performs the ADF reading as described above, it is necessary to move the optical reading system, which reads a document at a predetermined position, to the read position of the white reference member so as to generate shading data. If the optical reading system is moved every time a document is read, image data productivity is decreased. If shading data is continuously used without moving the optical reading system every time a document is read, accurate shading correction corresponding to the light intensity of the light source cannot be performed because the light intensity varies due to a change in the surface temperature of a tube of the light source.
For example, Japanese Patent Application Laid-open No. 2003-037717 discloses a technology in which the mean level of white reference of a second reference white plate, which is arranged outside the effective pixel area at the read position, is calculated in a sheet interval during which ADF reading is being performed, and shading data is corrected by using the ratio of the calculated mean level to the mean level that has been calculated in the previous sheet interval. According to the technology disclosed in Japanese Patent Application Laid-open No. 2003-037717, accurate shading correction corresponding to a change in the light intensity of the light source can be performed without generating shading data each time.
In the technology disclosed in Japanese Patent Application Laid-open No. 2003-037717, the presence of the second reference white plate used for correcting shading data being arranged outside the effective pixel area increases the size of the image reading apparatus and, because the necessity of enlarging an image sensor increases the costs for producing and running the image reading apparatus.
A possible method for solving the above problem is to arrange a roller-shaped white reference member used for correcting shading data in the background of the document read position. In this case, dirt may adhere to the white reference member used for correcting shading data when a document is passing therethrough; therefore, because of the dirt, the white level of the white reference member used for correcting shading data may have an abnormal value, and accordingly, shading data that has been corrected by using this white level also may have an abnormal value. As a result, there is a possibility of the occurrence of an abnormal image.
For example, Japanese Patent No. 4148655 discloses the technology in which an area corresponding to more than one revolution of a read roller is read in a sheet interval during which ADF reading is being performed and shading data is corrected by using data on the cleanest area. By using the technology disclosed in Japanese Patent No. 4148655, it is possible to prevent the white level of the white reference member used for correcting shading data from becoming an abnormal level due to dirt and to avoid the occurrence of an abnormal image.
In the technology disclosed in Japanese Patent No. 4148655, it is necessary to ensure that there is enough reading time to read an area corresponding to more than one revolution of the read roller in a sheet interval during which document reading is being performed; therefore, if the actual sheet interval is shorter than the reading time, image data productivity is decreased.